(i) Field of the Invention
This invention relates to a roll door assembly, more especially a roll door assembly for mounting in an open doorway of a commercial or industrial building.
(ii) Description of the Prior Art
Commercial and industrial buildings have large doorways or openings for passage of vehicles such as forklift trucks and other traffic. It is desirable to seal such openings closed when not in use in order to reduce loss of heat from a heated area to a non-heated or cold area, to maintain particular atmospheric conditions and temperatures in specialized environments, to maintain worker comfort and for other purposes.
Flexible doors have been developed for this purpose which hang down to close the opening but may be rolled up to permit access through the doorway or opening. In particular, the flexible roll door is supported on a rotatable shaft disposed across the top of the door opening and is unwound from or wound on the shaft to close or open the doorway. For this purpose the shaft is driven by a motor, the direction of rotation of the shaft determining whether the roll door is unwound from or wound on the shaft.
Since the door is necessarily of flexible material, to enable it to be rolled on and off the shaft, it is necessary to provide some means for tensioning the flexible material so that it is retained tau.. For this purpose it has previously been proposed to employ a heavy spring under tension to counterbalance the weight of the roll door and hold it taut.
Such springs display several disadvantages. In particular, the tensioning force applied by the spring is not uniform but varies with the load which it is counterbalancing, so that a uniform counterbalancing force is not applied throughout the vertical travel of the roll door.
Springs deteriorate with use and age and this also results in variation of the counterbalancing force.
Furthermore, a heavy spring maintained in a tensioned state represents a potential hazard in the event that the spring breaks free.